Steel having high resistance to corrosion by ammonia gas at high temperatures and pressures



United States Patent 3,362,855 STEEL HAVING HIGH RESISTANCE T0 CORRO- SION BY AMMONIA GAS AT HIGH TEMPERA- TURES AND PRESSURES Mitsuo Shimo, Nakano-ku, Tokyo, Atsuo Murata, Shibuya-ku, Tokyo, and Ryo Kokubo, Mizuhiko Nagakura, and Koji Sima, Neigum, Toyama Prefecture, Japan, assignors to Nissan Kagaku, Kogyo Kabushiki Kaisha, Tokyo, Japan No Drawing. Filed Sept. 1, 1964, Ser. No. 395,108 Claims priority, application Japan, Sept. 7, 1963, 38/ 17,540 6 Claims. (Cl. 148-16) The present invention relates to a method of protecting metallic apparatus from the corrosive effects of high-temperature, high-pressure NH gas by fabricating the apparatus from a stainless steel of austenitic structure comprising to 12% chromium and 17 to 40% nickel, the balance being mainly iron, when said apparatus is exposed to NH, gas of over 10% concentration under the conditions of pressure 50 to 350 kg./cm. and temperature 350 to 550 C.

Such protection may also be obtained by fabricating the apparatus from a stainless steel of austenitic structure comprising 5 to 12% chromium, 17 to 40% nickel, 0.5 to 4% copper or niobium, the balance being mainly iron.

The object of this invention is to offer an economical solution to the problem of the corrosion, chiefly through nitriding, of the metallic materials of apparatus when exposed to high-temperature, high-pressure NH gas, said solution lying in the adoption of a cheap, easily workable stainless steel with high anti-corrosive properties.

It has long been known that various irons and steels, when exposed to NH gas at over 400 C., will suffer serious corrosion through nitriding.

It has been observed that this nitriding corrosion of iron and steel by such high temperature NH gas will make considerable progress in both a synthetic NH atmosphere with an NH, concentration of a mere 2 to 20% and a normal pressure NH gas, as encountered in the practice of producing a nitriding steel.

In the very early stage of nitriding irons and steels, the surface nitriding layer represents a very hard, strong film of close texture, but as the nitriding process progresses, this layer transforms itself steadily into a fragile scale, finally falling off the body of metal.

Therefore, it is very important in the process of NH synthesis to choose an anti-nitriding material with low vulnerability to nitriding corrosion for the metallic members of apparatus coming into contact with high temperature NH gas or for the members of the nitriding box to be employed in the production of nitriding steel, and much research has been devoted to this subject.

This reasearch has shown that anti-nitriding property would be improved by increasing the nickel content of steel. Thus, Hastelloy, Inconel, etc., have come to be recommended as excellent materials for such purpose.

These high nickel alloys, however, are regarded as uneconomic because of extremely high expense and poor workability or weldability, although they may excel in resistance to nitriding.

As a matter of fact, 5 to 20% chromium stainless steels, e.g., 18 Cr*8 Ni, steel for NH synthesis, and 25 C-r-20 Ni steel for nitriding box have more often been used, in spite of the corrosion which results.

In the course of research on the use of a high-temperature, high-pressure NH gas of heavy concentration under pressures of 50 to 350 kg./cm. and at temperatures of 350 to 550 C., the present inventors have found that the nitriding corrosion in such high-temperature, high-pressure NH gas of high concentration is much more intense than in high-pressure NH gas of low concentration or in normal pressure NH gas of high concentration, and have realized that chemical production processes involving such high-temperature, high-pressure NH gas of high concentration will be hamstrung unless some new grade of stainless steel with excellent anti-nitriding property could be developed.

Even at 350 C. a relatively intense nitriding corrosion is in evidence in an NH gas of over 50% concentration with a pressure of more than 50 kg./cm. and in a pure NH gas with 100 kg./cm. 400 C., the nitriding corrosion of AISI 316, i.e., 18 Cr-14 Ni-2 Mo stainless steel, will amount to 1 mm./ year.

Such high nickel alloys as Hastelloy and Inconel are highly anti-nitriding to such high-temperature, high-pressure NH gas of heavy concentration, but as stated above, they have been found impractical because of high cost and poor workability.

Particularly, it is extremely difficult or even impossible to fabricate seamless tubes or high pressure vessels from these alloys.

The present inventors have succeeded in perfecting the present invention as the result of meticulous investigations of the relation between Cr and Ni contents and the resistance to nitriding of stainless steels having a far smaller Ni content than said high nickel alloys, with a view to developing a stainless steel which is strongly antinitriding and at the same time cheap and easy to work.

If a stainless steel of austenitic structure comprising 5 to 12% chrome, 17 to nickel and the rest mainly iron is employed to construct apparatus which is to be exposed to an NH, gas of over 10% concentration with pressure to 350 kg./cm. temperature 350 to 550 C., such steel will successfully resist corrosion by such hightemperature, high-pressure NH gas.

With over 12% chromium content, according to Tammanns law of active limit the elfect of chromium begins to prevail and the resistance to nitriding remarkably deteriorates. On the contrary, if the chromium content drops below 5%, this resistance is also seriously lowered for some unknown reason. Meanwhile, in the range from 5 to 12% chromium there is a general tendency for the resistance to nitriding to improve with a decreasing chromium content.

An increase in the percentage of nickel above 40% does not result in any substantial improvement in the resistance to nitriding, but the cost will increase and the workability will become poor. This will make the production of seamless tubes, high pressure vessels particularly difficult.

At less than 17% nickel, the resistance to nitriding will seriously deteriorate.

It is widely believed that any increase in the nickel content will increase the resistance to nitriding but this does not always hold. Take, for example, the case of Incoloy. This 20% chromium 35% Ni alloy is found to be remarkably inferior in resistance to nitriding, even poorer in this respect than 18 Cr-S Ni stainless steels. On the other hand, Hastelloy or Inconel, containing more than nickel, is found superior in its resistance to nitriding, in spite of a chrome content as high as 15%, but is expensive and hard to work.

An extremely significant discovery made by the present inventors in connection with the improvement in resistance to nitriding of steel is the fact that a stainless steel with high resistance to nitriding may be found among low chromium stainless steels with 5 to 12% chromium, even though they contain less than 40% nickel.

Of equal importance is the fact that a combination of appropriate contents of chromium and nickel must be selected within the limits of 5 to 12% chrome and 17 to 40% nickel, so that the structure of the stainless steel may be kept austenitic.

Even if the chrome content is held within to 12% and the nickel within 17 to 40%, the resistance to nitriding of a non-austenitic stainless steel will be very poor.

The stainless steel as proposed in the present invention is equally or more resistant than 18 Cr-S Ni to hydrogen embrittlement. No susceptibility to hydrogen embrittlement is observed as testified by the sectional structure of the test piece used in the test of Example 1.

Further, in the test with NH gas under a pressure of 50 to 350 kg./cm. with temperature 350 to 550 0., there is a consistent relationship between grade of stainless steel and the intensity of nitriding corrosion, though in the case of a pure NH gas with 100 kg./cm. 400 C., this relationship is less consistent.

The stainless steel of this invention is not inferior to such high nickel alloys as Hastelloy, Inconel, etc., in its resistance to nitriding under exposure to high-temperature, high-pressure NH gas of high concentration. Moreover, it is cheap and so easy to work that it can be utilized for fabrication of seamless tubes or high pressure vessels. Its welclability is also good; its tensile strength is over 45 kg/rnm. at 450 C., with no susceptibility to hydrogen embrittlemen t, which is as good as or better than 18 Cr- 8 Ni stainless steels.

As clear from the above explanation, the main feature of the present invention is the improvement on the method of protecting industrial apparatus from the corrosive infiuence of high-temperature, high-pressure NH gas by fabricating said apparatus from a stainless steel of austenitic structure comprising 5 to 12% chromium, '17 to 40% nickel, and the rest mainly iron. Another discovery through this invention is that partial replacement of iron with copper or niobium will further improve the antinitriding property of said stainless steel, the preferred addition of copper of niobium being at least 0.5% while an addition of over 4% is undesirable because it greatly impairs the workability of said stainless steel.

Thus another feature of this invention is the protection of the metallic materials of industrial apparatus from the influence of high-temperature, high-pressure NH gas by making that apparatus from a stainless steel of austeriitic structure with 5 to 12% chrome, :17 to 40% nickel, 0.5 to 4% copper or niobium, and the rest mainly iron.

Three examples of the first embodiment of the invention will now be described:

Example 1 Various test pieces were enclosed in a small autoclave, in which a pure NI-I gas was circulated under pressure 100 kg./cm. and temperature 400 C.; and after 1000 hours of exposure to said gas they were taken out, measured for a weight gain due to nitriding and observed for the surface condition, with the results as listed in the following table. 0

Test pieces were 3 X 30 x 50 mm. with austenitic struc- In the third column of the above table, the dashes mean that the fragile layer already formed on the surface fell off, resulting in a weight loss.

The degree of nitriding corrosion may be roughly estimated in terms of weight gain. In the case of a large weight gain, the surface nitriding layer will turn into a fragile scale and may fall ofi. In the case of a small weight gain, the nitriding process will progress relatively quickly in the early stages, but the nitriding rate becomes steadily more sluggish without causing transformation of the surface nitride layer into a scale.

From the above table it is obvious that the resistance to nitriding of what might be called the reverse 18-8 type stainless steel, that is, the opposite of 8 Cr18 Ni, one which has less Cr and more nickel is extremely good.

In fact, the effect of chromium content on the resistance to nitriding is so great that it may be called dominant.

It is an absolute necessity to keep the chromium content within the limits of 5 to 12%.

Example 2 Test Composition Nitriding corrosion Welding piece or name (thickness of nitriding after expolayer in mmJy.) sure test No. 4 8 Cr30 Ni..- 0.02 Possible Control A181 304 0. 1 Impossrble. Do A181 316 0.1 Do.

In this example, the nitriding corrosion of 8 Cr-30 Ni stainless steel was a mere fraction of that of the traditional A151 304 or A181 316. The 8 Cr-30 Ni steel exhibited a surface with metallic luster and was weldable after exposure to the harmful gas.

Therefore, if the proposed stainless steel in this invention is adopted for the metallic members of the parts coming into contact with an NH gas of over 10% concentration in the NH synthesizer, the synthesizer can be used for an extremely long period without any repair; and in case of necessity its repair is possible.

Example 3 The proposed 8 Cr-30- Ni stainless steel was employed as the material for a heating tube in which liquid ammonia was circulated under high pressure; the tube was heated from outside and the steel came into contact at the exit of tube with an NI-I gas with pressure 130 kg./ 0111. and temperature 450 C.

Liquid ammonia was delivered by the pump at a rate of 3 kg./hr. under a pressure of 130 kg./cm. into a seamless heating tube, 20 mm. out. dia., 10 mm. in. dia., 10 m. long, with an effective heating length of about 8 m.

The heating electric wire was of about SKW capacity and the voltage imposed on the wire was so regulated that the NH gas would have a temperature of 450 C. at the exit of tube.

After some 1000 hours of operation, the exit portion of the tube was cut off for inspection of the interior surface and the sectional surface. There was nothing wrong observed except a thin, firm nitriding film of metallic luster on the inside surface.

Three examples of the second embodiment of invention, i.e., the composition comprising copper or niobium as a third ingredient will now be described.

Example 4 Various test pieces were enclosed in a small autoclave, in which a pure NH gas was circulated under 100 kg./ cm. at 400 C. After a certain period of exposure to the gas, they were taken out, measured for weight gain due to nitriding, and observed for surface condition. with the following results.

Test pieces were 3 x 30 x 50 mm. and of austenitic structure.

Weight gain due to Test nitriding (mg/cm) pieces Composition Surface condition after 1,000 hrs. 500 hr. 1,000 hr.

N0. 1 (Jr- Ni-2 Cu-. 1.14 1. 63 Metallic luster. No. 2 5 Cr- Ni-2 Cu 0.80 1.32 Do. No.3. 8 (Zr-3 Ni-0.5 Nb 1. 06 1. 54 D0. Control 5 Cr-25 Ni 1.08 1. 61 Do.

The above table clearly shows that the stainless steel comprising 5 to 12% chromium and 17 to 40% nickel is itself an excellent anti-nitriding steel, but a small addition of copper with further improve its anti-nitriding property. 0.5 to 4% addition of copper will reduce the weight gain of steel due to nitriding to about /3 of that in a steel with no copper content, and will further strengthen the nitriding film with the metallic luster of surface being maintained for a long period.

The mecahnism of such improvement on the anti-nitriding property of stainless steel through a small addition of copper is not well elucidated; probably, copper may be negatively catalytic to nitriding.

Though not so much as copper, niobium, too, is eifective for improvement of anti-nitriding property.

The sectional structures of the test pieces in this example never exhibited any evidence of hydrogen embrittlement and exhibited a resistance to hydrogen embrittlement which is the same as that of the proposed stainless steel which constitutes the first embodiment of this invention and contains no copper or niobium.

Example 5 In an NH synthesizer with a plurality of catalyzer layers operating at a pressure of 300 kg./cm. test pieces were placed and held for about half a year nearly midway between catalyzer layers having a gas with about 13% NH concentration, the balance being nitrogen and hydrogen with a molar ratio 1:3, the gas temperature being 500 C.

The results of testing for resistance to nitriding were as indicated in the following table.

Corrosion due to Test pieces Composition or nitridiug (thiek- Welding after name ness of nitriding exposure test layer), mm./y.

No. 4 8 Cr- Ni-4 Cu 0.01 Possible. C0utrol S Cr-30 Ni 0. O2 0.

Do AISI 304 0.1 Impossible.

coming into contact with an NH gas of over 10% concentration in an NH synthesizer, the synthesizer can be used for an extremely long period without any repair; and in case of necessity its repair is possible.

Example 6 The proposed 8 Cr-30 Ni-3 Cu stain-less steel was employed as the material for a heating tube in which, under the same conditions and using the same device as in Example 3, liquid ammonia was circulated at 3 kg./hr. under pressure of 130 kg./cm. and heated from the outside by an electric wire of 5 kW. capacity in such manner that the NH gas temperature at the exit of the tube would be 450 C.

After about 1500 hours of operation, the exit portion of heating tube was cut off to inspect the interior surface and the sectional surface. There was nothing Wrong observed except a thin nitriding film of metallic luster on the inside surface.

What is claimed is:

1. In the process of exposing metallic articles to high pressure ammonia gas, the step of contacting said metallic articles with ammonia gas of over 10% concentration, at a pressure of 50-350 kg./cm. at a temperature of 350- 550 C., while having at least their exposed portions which contact said gas, formed from an alloy having improved corrosion and nitriding resistance, consisting essentially of, 5-12% chromium, 17-40% nickel, and the balance substantially all iron.

2. In the process of claim 1 the improvement wherein said metallic articles are containers.

3. In the process of claim 1 the improvement wherein said metallic articles are pressure vessels.

4. In the process of claim 1 the improvement wherein said metallic articles are formed from an alloy consisting essentially of 512% chromium, 17-40% nickel, 0.5-4% of a metal selected from the group consisting of copper and niobium, and the balance substantially all iron.

5. In the process of claim 4 the improvement wherein said metallic articles are containers.

6. In the process of claim 4 the improvement wherein said metallic articles are pressure vessels.

References Cited UNITED STATES PATENTS 1,704,087 3/1929 Hybinette -1305 1,724,299 8/1929 Mitchell 29l96.l 1,953,229 4/1934 Heron 75128 2,054,405 9/1936 Becret 75-128 2,174,025 9/1939 Wise 75128 2,507,698 5/1950 Dubois 75l28 FOREIGN PATENTS 220,710 8/ 1924 Great Britain.

OTHER REFERENCES Transactions of the American Institute of Mining and Metallurgical Engineers, vol. 113, 1934, pp. 143-456.

DAVID L. RECK, Primary Examiner. P. WEINSTEIN, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,362,855 January 9, 1968 Mitsuo Shimo et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the heading to the printed specification, lines 8 and 9, for "assignors to Nissan Kagaku, Kogyo Kabushiki Kaisha, Tokyo, Japan" read assignors, by direct and mesne assignments, of one-half to Nissan Kagaku Kogyo Kabushiki Kaisha and one-half to Nippon Yakin Kogyo Kabushiki Kaisha, both of Tokyo, Japan Signed and sealed this 29th day of April 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. IN THE PROCESS OF EXPOSING METALLIC ARTICLES TO HIGH PRESSURE AMMONIA GAS, THE STEP OF CONTACTING SAID METALLIC ARTICLES WITH AMMONIA GAS OF OVER 10% CONCENTRATION, AT A PRESSURE OF 50-350 KG./CM2 AT A TEMPERATURE OF 350550*C. WHILE HAVING AT LEAST THEIR EXPOSED PORTIONS WHICH CONTACT SAID GAS, FORMED FROM AN ALLOY HAVING IMPROVED CORROSION AND NITRIDING RESISTNANCE, CONSISTING ESSENTIALLY OF, 5-12% CHROMIUM, 17-40% NICKEL, AND THE BALANCE SUBSTANTIALLY ALL IRON. 